Many of the world's large cities today have bad air quality.[1] Even 2,000 years ago, the Romans were complaining about the bad air in their cities. At that time, the air was thick with smoke from fires and the smell of sewers.[1] Air pollution has been a danger to human health and Earth's many ecosystems for a long time.

It has many pollutants (things that pollute the air) from natural sources. These pollutants include dust, sea salt, volcanicashes and gases, smoke from forest fires, pollen, and many other materials.[1] In fact, there are many more natural pollutants than pollutants that humans make.[1] However, humans have adapted to most of these natural pollutants.[1]

Ash from volcanic eruptions is an example of primary pollutants

Air pollution may be personal, occupational or community air pollution.

Air pollution is usually described as either primary pollutants or secondary pollutants. Primary pollutants are pollutants that are put directly into the air by humans or natural sources. Examples of primary pollutants are exhaust fumes (gas) from cars, soot from smoke, dust storms and ash from volcanic eruptions (as seen in the picture on the left).[1]

Secondary pollutants are pollutants that are made from chemical reactions when pollutants mix with other primary pollutants or natural substances like water vapor.[1] Many secondary pollutants are made when a primary pollutant reacts with sunlight. Ozone and smog are secondary pollutants. Ozone is a gas that stops harmful ultraviolet rays from the sun. When it is near the ground, though, it can poison people and other organisms.[1]

There are many problems caused by air pollution, like greenhouse effect and acid rain

Human-made air pollution comes from many things. Most air pollution made by humans today is because of transportation. Cars, for instance, make about 60% of the human-made air pollution. The gases inside car exhaust, like nitrogen oxide, make smog and acid rain.

Human-made air pollution is also caused when humans set their farmlands or forest on wildfires producing soot ( a black powder composed mainly of carbon, produced when coal, grasses, wood etc. is burned ) from smoke, that can affect people and other living when exposed to the atmosphere.

Air pollution is not only on the outside. Homes, schools, and buildings can also have air pollution. Sometimes the air inside a building is even worse than the air outside.[1] Many things which humans use every day can pollute the air. Compounds inside carpets, paints, building materials and furniture also pollute the air, especially when they are new.

In buildings where the windows are tightly shut to stop air leaks, the air inside can be polluted more than the air outside.[1]

Acid precipitation is precipitation, like rain, sleet, or snow, that contains acids from air pollution. When fossil fuels are burned, they let out oxides into the air. When these oxides mix with water in the atmosphere, they make acid, which fall as precipitation.[1] Acid precipitation can kill living things like fish and trees, by making the place where they live too acidic. Acid rain can also damage buildings made of limestone and concrete.

A global concern is the greenhouse gases and the hole in the ozone layer in the stratosphere. The Earth's ozone layer protects life from the Sun's harmful ultraviolet rays, but in the 1970s, scientists found out that some chemicals let out into the atmosphere makes the ozone turn into oxygen. This lets more ultraviolet rays reach the Earth. During the 1980s, scientists found that the ozone layer above the South Pole had thinned by 50 to 98 percent.

On March 17, 1992, in Mexico City, all children under the age of 14 could not go to school because of air pollution. This does not often happen, but being exposed to air pollution every day can make people have many health problems. Children, elderly (old) people, and people with allergies especially, can have a lot of problems because of air pollution. Studies from the University of Birmingham showed that deaths because of pneumonia and air pollution from motor vehicles like cars are related.[2]

The World Health Organization said that 2.4 million people died because of the direct problems of air pollution.[3][4] Some of the problems include:[1]

In India in 2014, it was reported that air pollution had cut crop yields in the most affected areas by almost half in 2010 when compared to 1980 levels. There can also be increased yield of crops due to some air quality conditions.

Stuff in the air Standard air quality modelling procedure for industrial sources.

Wiki on Atmospheric Dispersion Modelling. Addresses the international community of atmospheric dispersion modellers — primarily researchers, but also users of models. Its purpose is to pool experiences gained by dispersion modellers during their work.